The present invention relates generally to the calibration of a polarimeter and more particularly to a method and an apparatus to re-calibrate a polarimeter without the need for an electromagnetic wave with known polarization.
All polarimeters, which are instruments for measuring the polarizations of electromagnetic waves, have to be calibrated. After calibration, polarimeters drift over time. They have to be regularly re-calibrated especially when they are making accurate measurements. In a typical re-calibration, a polarimeter measures an electromagnetic wave with known polarization generated by a standard source. A relationship is then calculated to adjust the measured signals so that they indicate the polarization of the wave. Unfortunately, any practical standard source drifts over time. Unless the source is tuned back to its correct value, which can be a tedious task, the drifted source leads to erroneous recalibrations.
The standard source not only has to be stable, its wavelength has to be known. After re-calibration, the polarimeter can accurately measure the polarization of a wave with nearly the same wavelength as the standard source. If the wavelength is substantially different than that of the standard source, the accuracy of the measurement is significantly degraded. Thus, to make an accurate polarization measurement, the wavelength of the wave to be measured must be as close as possible to the wavelength of the standard source.
There is a need for a way to re-calibrate a polarimeter with a method that does not depend on a stable standard source. The re-calibrated polarimeter should be able to measure the polarization of waves without the need for knowing their wavelengths accurately. The re-calibration process should be fast and easily accomplished especially when the polarimeter is used for making accurate measurements.